Actor from a programme called Diff'rent Strokes back in the 70/80s. Gary played an adopted black child, very streetwise, hip etc who had been taken in by an upper class white male and his older (Then the Gray Character) white son. The family were clearly rich, which suited the "poor black kid" and the obvious implications of a whole white adult household adopting a young black child seemed to be OK in those non PC days. Gary got too "big for his boots" eventually as stardom and drugs/drink took over his life. A born loser in every respect IMHO.
"Whatchoo talkin' 'bout" was his catch phrase whenever any white man logic was applied to his misdeeds.
Wiki tells us that:
Coleman was born in Zion, Illinois. He suffers from a congenital kidney disease causing nephritis (an autoimmune destruction of the kidney), which halted his growth at an early age, leading to a small stature (4 ft 8 in; 1.42 m). He has undergone two kidney transplants, one in 1973 and one in 1984, and requires daily dialysis.